Thaw, peel and devein the prawns. Wash the prawns in lemon water and pat dry; provide.
Grind the garlic, onions, coriander and the cili pepper with a little salt in a mortar. Place the prawns in half of this spice paste with a tablespoon of olive oil and marinate for at least 1 hour.
Fry the diced tomatoes in a little olive oil with the other half of the paste. Add the coconut milk, cover the saucepan and simmer for 20 minutes.
Add the prawns with the marinade and the pepper rings and cook for another 5 minutes over low heat. Season with salt and pepper and serve with basmati or Thai rice.